When people hear that I'm an advocate of high yield farming to feed the world and protect the environment, assertions of farm runoff into the rivers are raised to support charges against modern farming methods. Urban dwellers, even some of my rural neighbors, tell me their concerns about large-scale farming ruining our rivers "because the rivers are muddy." They worry about even more soil erosion as farmers gear up to double food production over the next 40 years to feed a peak population of 9 billion people.

Certainly, the rivers in the world's farming areas run brown. Muddy rivers generally mean the surrounding soils are good enough to farm. But the farmland sustains high yields despite the brown rivers. The mountain streams produce no food—even though the water coming down the mountainside travels at much higher and more dangerous speeds and run crystal clear. Why? The soil from the mountainsides has mostly eroded long since.

Fortunately, you don't have to just take my word for that. A research team sponsored by Minnesota corn and soybean farmers just carried out an airborne laser scanning study of the Minnesota River above Mankato, MN. The study found that 56–95 percent of the sediment in the river came from the natural erosion along the riverbanks—which has been going on for centuries.

Dr. Satish Gupta of the Minnesota Department of Soil, Water and Climate was the lead author on the study. He says, "Some of these [river] banks are 150 feet high. They are very steep, not very stable, and they slough into the river." Gupta also emphasized that the sediment load in any farm-country river will be a combination of bank erosion and runoff from the farm fields. The proportions vary with the soils, slope, rainfall patterns and farming systems. Thanks to the laws of hydraulics, however, any stream will get enough sediment to slow itself down, one way of the other—as it flows brown.

Dr. Gupta notes that in addition to bank erosion, the Minnesota River has also been impacted by a Corps of Engineers dredging program. The Corps takes 20,000 cubic yards of sediment per year out of the river to maintain a nine-foot depth for barges and towboats. The dredging makes the river flow faster and straighter. So does the extra water from urban rooftops, streets, parking lots and airports running into the river instead of infiltrating the surrounding soils. What happens to the dredged sediment? Beneficial public uses include wetland creation, bird nesting creation, and upland habitat development.

Even though the Minnesota River study show up to 90 percent of the sediment coming from bank erosion, best-farming practices are still helpful in minimizing crop and soil losses. No-till farming, contour farming, grassed waterways and buffer strips at field edges all help reduce sediment loss. Fencing cattle from the creeks has also become a popular conservation policy in many areas (including my rural Shenandoah Valley.)

Continuous research and innovation has made today's farmers the most sustainable in history. Their high crop yields mean they need to farm less cropland to supply food demands. They restore the soil nutrients taken up by the growing crops with chemical fertilizers. This keeps the plant root structures strong, so they resist erosion. No-till farming by itself can reduce soil erosion from the fields by 65–95 percent. But don't expect to ever see crystal clear rivers in good farming country.

Dennis T. Avery, a senior fellow for the Hudson Institute in Washington, DC, is an environmental economist. He was formerly a senior analyst for the U.S. Department of State. He is co-author, with S. Fred Singer, of Unstoppable Global Warming Every 1,500 Years. Readers may write him at PO Box 202, Churchville, VA 24421 or email tocgfi@hughes.net.